This invention relates to a bubble column which can be operated using countercurrent flow and comprises horizontally disposed perforated trays in the middle part thereof and the use thereof for carrying out gas-liquid reactions. One use is for the oxidation stage of the anthraquinone process for the production of hydrogen peroxide.
Bubble columns are columnar vessels in which a gas in the form of bubbles comes into contact with a liquid, wherein substances are mostly transferred from one phase into the other phase. Accordingly, bubble columns are used for chemical reactions between components in a liquid phase and components in a gaseous phase. In order to intensify mass transfer between the phases and to reduce back-mixing effects, a plurality of perforated trays disposed one above another can also be used in bubble columns (Ullmann""s Encyclopedia of Industrial Chemistry 5th Ed. (1992), Vol. 24, 276-278).
The perforated trays of large-scale industrial bubble columns, namely those with a diameter of at least 1 m, are usually sieve plates with a hole diameter between 2 and 5 mm or dual flow trays with a hole diameter of up to 20 mm. Grids with a thin layer of conventional packing material situated thereon are also used instead of sieve plates. The space-time yield of gas-liquid reactions is strongly dependent on the gas content in the gas-liquid mixture flowing through the column. When employing bubble columns with the aforementioned sieve plates, it has not proved possible to increase the gas content above certain limiting values, and the space-time yield has thereby been limited. Therefore, there have been many attempts to increase the space-time yield by means of other built-in components and/or by using special injection means for the gas. However, the construction of bubble columns is made considerably more costly on an industrial scale by the use of the built-in components mentioned above, for instance static mixers.
DE 694 03 618 T2, a translation of EP 0,659,474 B1, teaches a method for bringing a gas current into contact with a liquid phase and teaches a device for this purpose. The device includes a column with perforated sieve plates in which the total surface area of the perforations is between 1/40 and 1/300 of the cross section available for perforations. The level of the liquid layer maintained on the sieve plates, which is adjusted, e.g., by means of weirs, is preferably in a range of 200 to 600 mm. The cross-sectional area of the individual perforations is in a range of 0.5 to 3.5 mm2.
Austrian patent 236,346 teaches specially perforated trays for columns such as those used for distillation methods and absorption methods. In addition to openings having vertical walls, the trays additionally contain a small number of openings with walls inclined at an angle to the main surface. The cross-sectional area of the openings is 0.155 to 31.7 mm2 and, according to the example, the surface is 0.63 mm2. During operation, a liquid flows over the trays. The patent document does not teach operating the column as a bubble column.
DE-AS 10 28 096 teaches a method for the continuous reaction of finely distributed solid bodies with liquids and/or gases. A column, operated with cocurrent flow, entirely filled with liquid and with sieve plates having perforations with a diameter of less than 1 mm, is used. A gas cushion inhibits the passage of liquid. The column has no devices for enabling countercurrent operation.
The bubble-column cascade reactor according to DE-OS 21 57 737 is essentially the same as the previously evaluated reactor. The entire free perforation area is preferably less than 5% of the reactor cross section and the perforation diameters are, according to the examples, 2 and 4 mm (=3.14 to 12.56 mm2). No suggestions of countercurrent operation and/or devices to this end are disclosed by the document.
One large-scale industrial process based on a gas-liquid reaction is the oxidation stage in the anthraquinone process (AO process) for the production of hydrogen peroxide. As is known, this process comprises a hydrogenation stage, an oxidation stage and an extraction stagexe2x80x94a review is given in Ullmann""s Encyclopedia of Industrial Chemistry 5th Ed. (1989), Vol. A13, 447-457. In the hydrogenation stage, a reaction medium which is based on one or more 2-alkylanthraquinones and/or tetrahydro derivatives thereof, and which is dissolved in a solvent system, is partially hydrogenated to form the corresponding hydroquinones and, in the oxidation stage, the hydroquinones contained in the hydrogenated working solution are re-oxidized to quinones by a gas containing O2, generally air, with the formation of hydrogen peroxide. The reaction in the oxidation stage should be as quantitative as possible with the avoidance of decomposition reactions of components of the working solution. Moreover, it should consume as little energy as possible and it should be capable of being conducted with a high space-time yield.
In the AO process, oxidation is first conducted in gasification towers disposed in series, using fresh air in each case. This is both costly on an industrial scale and relatively uneconomic. According to U.S. Pat. No. 3,073,680, the rate of oxidation can in fact be increased by maintaining defined bubble sizes, which can be obtained by means of fine-pored gas distributor elements such as frits, and by maintaining defined conditions of cross-sectional loading. However, problems arise with the separation of the resulting foam and with gas-liquid phase separation.
According to German patent specification 20 03 268, the aforementioned problems associated with the AO process can be solved by means of an oxidation column which is subdivided into two to six sections. In each section of this column, the working solution and the oxidizing gas are passed from the bottom to the top in cocurrent flow, but in the column as a whole the gas and the liquid move in countercurrent flow in relation to each other. In order to achieve intimate mixing, the individual sections contain suitable built-in components such as sieve plates or meshes, or are packed with packing elements.
In an attempt to reduce the pressure drop in the aforementioned cascade-type arrangement of columns, European patent specification 0 221 931 proposes that oxidation be conducted in a tubular cocurrent reactor which contains no built-in components apart from a special gas distributor element. This gas distributor element results in the formation, from the working solution and the oxidizing gas, of a system in which bubbles are inhibited from coalescing and which has a high gas content. If the gas content is too high and/or if the gas bubbles are particularly small, problems can arise with gas-liquid separation. It has been shown in practice that the specific reactor volume to be gasified (in m3 per ton H2O2) is quite large, which results in a reduced space-time yield and also results in a high hold-up of costly working solution.
The object of the present invention is to provide a bubble column which can be operated with countercurrent flow and which includes perforated trays with which gas-liquid reactions can be conducted with a higher space-time yield than when using columns comprising customary sieve plates. The bubble column should be of simple construction. A further object is oriented towards the use of the bubble column in the oxidation stage of the AO process for the production of hydrogen peroxide, wherein the conversion is improved compared with known processes, the space-time yield with respect to the reactor volume and the volume of working solution is improved, and the formation of a gas-liquid mixture which is difficult to separate is avoided.
This object is achieved by a bubble column comprising a columnar vessel (1) having a bottom (3), middle (2) and top part (4), one or more perforated trays (5) horizontally disposed in the middle part and with a distribution of holes which is essentially uniform over the cross section of the column, and devices for feeding and discharging a liquid phase (9 and 10) and a gas phase (11 and 12) in order to operate the bubble column with countercurrent flow, which is characterized in that the cross-sectional area of the individual holes is 0.003 to 3 mm2, the open area of the trays is 3 to 20% and in that the liquid zones formed in the operating state above and below a tray communicate with each other via a passage (6) for throughflow of liquid.
Compared with bubble columns comprising conventional sieve plates, the bubble columns according to the invention are characterized by trays with fine holes or fine slits. The trays preferably contain holes with a cross-sectional area of 0.01 to 1 mm2, particularly 0.05 to 0.05 mm2. The open area preferably falls within the range from 3 to 15%, preferably 3 to 10%, most preferably 3 to 7%. The shape of the holes is arbitrary, but the holes are usually of round, triangular to semi-elliptical, or slit-shaped construction. The trays comprising fine holes or fine slits can be constructed as complete column trays, but usually consist of a supporting grid and a plate which is fixed thereon, which comprises fine holes or fine slits and which is of the desired plate thickness and degree of perforation. Plates of this type comprising fine holes or slits are in fact used in sieving and filtration technology and as fluidizing bases in fluidized bed technology. Their use as trays in bubble columns has never been considered previously, however.
As determined by their manufacture, the holes in the plates are preferably of tapered construction in the direction of passage of the gas and/or the holes are inclined in addition for the purpose of achieving a directed flow during passage of the gas. A directed flow can additionally be effected by the scale which is formed on the surface of the plate due to the manufacture thereof.
The bubble column is divided into several zones by the finely perforated trays in the middle part of the bubble column which are filled with liquid or a liquid-gas mixture in the operating state, with the exception of a thin gas cushion directly below the trays. In order that smooth operation is assured during countercurrent operation, the bubble column comprises at least one tubular or well-shaped liquid passage (6) per tray, between adjacent zones, so-called xe2x80x9cdowncomersxe2x80x9d. These passages, which advantageously inlet directly on the tray, thus avoiding any need for a weir, dip into the liquid of the zone located below the particular tray or are connected to it. The passages are designed so that no gas flows through them in the operating state. This is achieved, e.g., in that the downcomers, advantageously in the form of round tubes or segment-like wells arranged on the perforated trays with an appropriate free cross section, empty into a dip pocket. Alternatively, outside pipes connecting two adjacent zones at a time can also be used as downcomers.
The bubble column is usually designed in such a manner that it can be operated using countercurrent flow, during which a liquid is discharged at the top and a gas is supplied at the bottom. If the device (6) for the passage of liquid is not present, the bubble column can also be operated using cocurrent flow, during which a mixture of liquid and gas flows upward from below.
The tray interval in the bubble column of the invention is dependent on the specific problem posed and the diameter-to-height ratio of the bubble column. The tray interval is in general in a range of 0.1 to 10 times, especially 0.05 to 5 times the tray diameter. In large-scale industrial bubble columns like those used, e.g., in the use according to the invention for the production of hydrogen peroxide, the tray interval of the trays with fine holes or fine slits is preferably in a range of 0.05 to 2 times the tray diameter.
Aside from the trays discussed above, which are preferably provided, for countercurrent operation, with at least one tubular or well-shaped liquid passage each, the middle part of the column can be free of built-in components. However, according to a preferred embodiment it is also possible to arrange heat exchangers between individual trays, advantageously plate heat exchangers with vertically positioned plates. Such bubble columns provided with trays with fine holes and with heat exchangers can be used with particular advantage to carry out gas-liquid reactions with a high reaction enthalpy. The bubble columns of the invention can be equipped in a manner customary for one skilled in the art for operation using cocurrent or countercurrent flow, preferably countercurrent flow. A cascade design is also possible.
As can be seen from the examples according to the invention and from the comparative examples, extraordinary, unforeseeable advantages are achieved due to the design according to the invention comprising perforated trays in the bubble column:
gasification of a liquid situated above the plates comprising fine holes or slits is extremely uniform;
small bubbles with a narrow range of diameters are produced uniformly over the entire cross-section of the bubble column;
the efficiency of the intensive mass transfer which is due to the trays enables the specific gasification volume (=effective reactor volume) to be reduced compared with bubble columns comprising sieve plates;
the gas content of the gas-liquid mixture which can be attained in practice is significantly greater than the gas content which can be obtained when using conventional sieve plates and in other gasification techniques, without this resulting in problems in gas-liquid phase separation;
the mass transfer area, and the extent of mass transfer which is achieved therewith, is very high;
compared with conventional columns, the hold-up of the liquid phase is significantly reduced; in particular, this is a considerable advantage if the liquid phase is a costly multi-component mixture, for instance the working solution of the AO process;
a higher reaction conversion is achieved per m3 of reactor volume compared with competing processes;
a higher reaction conversion is achieved per m3 of liquid phase (e.g. the working solution in the AO process);
the pressure drop across the trays is about 300 to 500 Pa (3-5 mbar) per tray, and is therefore low compared with the hydrostatic pressure drop in the column; a gas cushion with a depth of only 1 to 5 cm is formed under the trays, so that practically the complete apparatus volume (=the middle part of the column) can be utilized for the reaction.